In recent years, organic EL devices have been under intensive investigation. One such organic EL device basically includes a transparent electrode (anode) of tin-doped indium oxide (ITO), etc. A thin film is formed on the transparent electrode by evaporating a hole transporting material such as tetraphenyldiamine (TPD). A light emitting layer of a fluorescent material such as an aluminum quinolinol complex (Alq.sup.3) is deposited on the hole transporting thin film. An electrode (cathode) is formed thereon from a metal having a low work function such as magnesium or Mg. This organic EL device attracts attentions because it can achieve a very high luminance ranging from several hundreds to tens of thousands cd/m.sup.2 with a voltage of approximately 10 volts.
A cathode considered to be effective for such organic EL devices is made up of a material capable of injecting more electrons into the light emitting layer. In other words, the lower the work function of a material, the more suitable is the material as the cathode. Various materials having a low work function are available. For instance, the materials which are used as the cathodes of organic EL devices generally include MgAg, and AlLi disclosed in JP-A 4-233194. The reason is that the production process for organic EL devices relies mainly upon evaporation making use of resistance heating, and so the evaporation source used therewith is as a matter of course limited to one having a high vapor pressure at low temperatures. A cathode produced by such an evaporation process making use of resistance heating is poor in its adhesion to the interface between the cathode and the organic layer. Consequently, non-light emitting spots called dark spots occur on pixels just after production. The dark spots become large as the device is driven, and so become a leading factor that governs the service life of the device.
The aforesaid JP-A 4-233194 describes that alkali earth metals, and rare earth metals having a low work function are suitable for cathodes, and adds that alkali metals are excluded from electrode materials because they are too unstable for use as electrodes.
On the other hand, JP-A 4-212287 discloses that relatively stable alloys comprising alkali metal elements and other metals and having a low work function are usable as cathodes. More specifically, the cathode alloys disclosed therein are composed mainly of alkali metal elements such as Li, Na, and K, and more stable other metals such as Mg, Al, In, Sn, Zn, Ag, and Zr. Here, the alkali metal elements such as Li, Na, and K are stabilized by alloying with more stable metals such as Mg, Al, In, Sn, Zn, Ag, and Zr. However, the examples given therein reveal that alloying is carried out by co-evaporation. It is thus believed that the aforesaid stabilizing metals, too, are limited from the standpoint of vapor pressure. Furthermore, the organic EL element set forth therein fails to solve the aforesaid problem because the constituting films are all formed by evaporation.